A problem particular to mobile radio dispatch systems is the distance between an operator dispatch console and RF transmission and reception equipment. This problem arises because of the necessity to position the RF equipment to ensure geographic coverage.
This problem has traditionally been addressed by remoting the connection between the dispatch console and the RF equipment over leased lines, using various multiplexing methods to combine the voice signals and the control signals over a minimum number of lines. For instance, one common method involves multiplexing a DC current over the AC voice signal, with the magnitude and polarity of the current representing a specific function that the RF equipment must perform. Another common method involves sending a precise tone sequence composed of audible tones, with the frequency of the tones again representing a specific control function. The method used depends on the type of the equipment.
In all cases, the control functions relayed using those methods pertain to basic RF equipment control. Functions pertaining to the exchange of data between the dispatch console and remote subscribers over the RF network, such as the transport of unit ID or paging addresses, are achieved by overlaying a second multiplexing protocol, such as a low bit-rate modem protocol or a second tone sequence protocol, over the voice channel established between the dispatch console and the subscriber units. This causes the following problems: first, the voice channel is already non-ideal due to the underlying signal multiplexing method (this is especially true with the tone sequence method, that reserves a portion of the audible spectrum), and second, the dispatch console needs to implement every variant of the subscriber unit data protocols.
Recently, there has been interest in the use of Voice over Internet Protocol methods to replace the leased lines used to remote the dispatch console to RF equipment link. Some commercial equipment exists today. However, this equipment aims only to replace the voice channel and multiplexed current or tone signalling with packetized voice signals and embedded control sequences, while maintaining the data signalling as a low bit-rate coding method or second tone sequence method over the packet voice channel. A dispatch console is then required to separate the control signalling from the voice signalling. This method offers the following drawbacks. First, it restricts the use of voice codecs to the types of codecs that will accurately reproduce the low bit-rate data or tone sequence. This poses a significant problem as the modern codecs are designed to model to speech tract and as such are not optimized to pass modulated data. This requires a codec using 32 kbps or more bandwidth, such as a G.726 ADPCM codec and enough bandwidth throughout the whole system to carry the combined signals. Secondly, the dispatch console must still implement all variants of the data protocols. This implies that a different interface card may be needed for each protocol.